Sorry for the noise,
i found the answer.
i used the *expiration-action-handler* defvar,
to redefine my own action : a redirect to an "Expired Session" page.
I found too that the default timeout for sessions
is not the weblocks:*request-timeout* defvar
but the hunchentoot:*session-max-time* (1800 seconds).
May be it will help.
Regards